This invention relates to (i) a polymer comprising specific recurring units, (ii) a resist composition comprising the polymer as a base resin, (iii) a patterning process using the resist composition, (iv) a novel tetrahydrofuran compound useful as a monomer to form the polymer, and (v) a method for preparing the tetrahydrofuran compound.
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 xcexcm or less.
For resist materials for use with a KrF excimer lasers, polyhydroxystyrene having a practical level of transmittance and etching resistance is, in fact, a standard base resin. For resist materials for use with ArF excimer lasers, polyacrylic or polymethacrylic acid derivatives and polymers containing aliphatic cyclic compounds in the backbone are under investigation. All these polymers have advantages and disadvantages, and none of them have been established as the standard base resin.
More particularly, resist compositions using derivatives of polyacrylic or polymethacrylic acid have the advantages of high reactivity of acid-decomposable groups and good substrate adhesion and give relatively satisfactory results with respect to sensitivity and resolution, but have extremely low etching resistance and are impractical because the resin backbone is weak. On the other hand, resist compositions using polymers containing alicyclic compounds in their backbone have a practically acceptable level of etching resistance because the resin backbone is robust, but are very low in sensitivity and resolution because the reactivity of acid-decomposable protective groups is extremely low as compared with those on the (meth)acrylic polymers. Since the backbone of the resin is too robust, substrate adhesion is poor. These compositions are thus impractical as well.
Both the (meth)acrylic and alicyclic backbone systems commonly have the problem of pattern disruption due to swelling of resist film. Resist compositions based on these systems have been designed so as to improve their resolution performance by increasing the difference in dissolution rate before and after exposure, and as a consequence, they eventually become highly hydrophobic. Highly hydrophobic resist compositions, when applied as a film and processed with a developer, can maintain the film tenaciously in unexposed regions and allow the film to be instantaneously dissolved in over-exposed regions, while relatively-broad exposed regions therebetween allow penetration of the developer, but are kept undissolved, that is, swollen. At the very small pattern size for which an ArF excimer laser is actually used, those resist compositions which allow adjacent pattern strips to be joined together and disrupted on account of swelling are rejected. While a finer pattern rule is being demanded, there is a need to have a resist material which is not only satisfactory in sensitivity, resolution, and etching resistance, but fully restrained from swelling.
Therefore, an object of the present invention is to provide (i) a polymer having improved reactivity, robustness and substrate adhesion as well as minimized swell during development, (ii) a resist composition comprising the polymer as a base resin, which has a higher resolution and etching resistance than conventional resist compositions, (iii) a patterning process using the resist composition, (iv) a novel tetrahydrofuran compound useful as a monomer to form the polymer, and (v) a method for preparing the tetrahydrofuran compound.
It has been found that novel polymers comprising recurring units of the following general formula (1-1) or (1-2) and having a weight average molecular weight of 1,000 to 500,000, which are produced by the method to be described later, have improved reactivity, robustness or rigidity and substrate adhesion as well as an appropriately high solubility and minimized swell in a developer; that a resist composition comprising the polymer as the base resin has a high resolution and etching resistance; and that this resist composition lends itself to precise micropatterning.
In a first aspect, the invention provides a polymer comprising recurring units of the following general formula (1-1) or (1-2) and having a weight average molecular weight of 1,000 to 500,000. 
Herein each of R1, R2, R3 and R4 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, or a pair of R1 and R2 and a pair of R3 and R4 taken together may form a ring wherein each said pair is a straight, branched or cyclic alkylene group of 2 to 15 carbon atoms and k is equal to 0 or 1.
In one preferred embodiment, the polymer includes, in addition to the recurring units of formula (1-1), recurring units of the following general formula (2-1). 
Herein R5 is hydrogen, methyl or CH2CO2R7; R6 is hydrogen, methyl or CO2R7; R7 which may be identical or different in R5 and R6 is a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms; R8 is an acid labile group; R9 is selected from the class consisting of a halogen atom, a hydroxyl group, a straight, branched or cyclic alkoxy, acyloxy or alkylsulfonyloxy group of 1 to 15 carbon atoms, and a straight, branched or cyclic alkoxycarbonyloxy or alkoxyalkoxy group of 2 to 15 carbon atoms, in which some or all of the hydrogen atoms on constituent carbon atoms may be substituted with halogen atoms; Z is a single bond or a straight, branched or cyclic (p+2)-valent hydrocarbon group of 1 to 5 carbon atoms, in which at least one methylene may be substituted with oxygen to form a chain-like or cyclic ether or two hydrogen atoms on a common carbon may be substituted with oxygen to form a ketone; kxe2x80x2 is 0 or 1; and p is 0, 1 or 2.
In another preferred embodiment, the polymer includes, in addition to the recurring units of formula (1-1), recurring units of the following general formulae (2-1) and (3). 
Herein kxe2x80x2, p and R5 to R9 are as defined above, Y is xe2x80x94Oxe2x80x94 or xe2x80x94(NR10)xe2x80x94, and R10 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms.
In a further preferred embodiment, the polymer includes, in addition to the recurring units of formula (1-1), recurring units of the following general formula (4) alone or in combination with recurring units of the following general formula (2-1), and recurring units of the following general formula (3). 
Herein kxe2x80x2, p, R5 to R9 and Y are as defined above, R5xe2x80x2 is hydrogen, methyl or CH2CO2R7xe2x80x2, R6xe2x80x2 is hydrogen, methyl or CO2R7xe2x80x2, R7xe2x80x2 which may be identical or different in R5xe2x80x2 and R6xe2x80x2 is a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, and R8xe2x80x2 is an acid labile group.
In a still further preferred embodiment, the polymer includes, in addition to the recurring units of formula (1-2), recurring units of the following general formula (2-2). 
Herein kxe2x80x2, p and R5 to R9 are as defined above.
In a second aspect, the invention provides a resist composition comprising the polymer defined above.
In a third aspect, the invention provides a process for forming a resist pattern comprising the steps of applying the resist composition onto a substrate to form a coating; heat treating the coating and then exposing it to high-energy radiation or electron beams through a photo mask; and optionally heat treating the exposed coating and developing it with a developer.
In a fourth aspect, the invention provides a tetrahydrofuran compound of the following general formula (5). 
Herein R1, R2 and k are as defined above.
The tetrahydrofuran compound is preferably of the following general formula (6). 
Herein R1 and R2 are as defined above.
In a fifth aspect, the invention provides a method for preparing a tetrahydrofuran compound of the following general formula (5), comprising the step (a) of subjecting a diol compound of the following general formula (7) to intramolecular dehydration. 
Herein R1, R2 and k are as defined above, and step (a) represents dehydration reaction.
In an alternative embodiment, a tetrahydrofuran compound of the following general formula (5) is prepared by converting a diol compound of the following general formula (7) to a compound having an eliminatable group X of the following general formula (8-1) and/or (8-2), and treating the compound with a base for cyclization. 
Herein R1, R2 and k are as defined above, X is halogen, alkylsulfonyloxy or arylsulfonyloxy, step (a) represents halogen-substitution reaction, alkylsulfonylation reaction or arylsulfonylation reaction on hydroxyl, and step (b) represents HX-eliminating reaction.
The polymer comprising recurring units of formula (1-1) or (1-2) has high robustness due to the inclusion of a bridged aliphatic ring in the backbone. As opposed to a polar group introduced at a site spaced apart from the backbone, a highly polar ether site positioned within the backbone allows for rapid transition from the penetration of a developer to the dissolution of the resin, during which period little swelling occurs. In addition, since small amounts of the recurring units of formula (1-1) or (1-2) introduced are sufficient to fully exert the swell suppressing effect and the substrate bonding effect, the units for establishing differential dissolution rate such as blocked carboxylic acid units can be introduced in relatively large amounts, eventually enhancing sensitivity and resolution. Therefore, a resist composition using the inventive polymer as a base resin satisfies all the performance factors of sensitivity, resolution and etching resistance, is fully restrained from swelling upon development, and is thus very useful in forming micropatterns.